A paleostriatal-thalamic-telencephalic path in pigeons.

The efferent connections of the thalamic nucleus dorsointermedius posterior were studied in the pigeon. In pigeons, this thalamic nucleus receives both paleostriatal (basal ganglia) and cerebellar input and has been compared to nuclei of the mammalian thalamic ventral tier on this basis. The present results show that neurons of nucleus dorsointermedius posterior project upon a crescent-shaped neuronal field within the neostriatum intermedium as well as upon the ventral paleostriatum and nucleus intrapeduncularis. The relationship of the neostriatum intermedium field to avian motor pathways is discussed and it is suggested that nucleus dorsointermedius posterior thalami of birds may play a role comparable to that of the mammalian ventral tier nuclei in the neural control of motor functions. The efferent projections of dorsointermedius posterior were contrasted with those of adjacent structures within the avian posterior thalamic complex. Neurons within nucleus dorsomedialis posterior appear to be related to the avian limbic system and project to a neuronal field within the rostromedial portion of the neostriatum intermedium. Neurons in nucleus dorsolateralis posterior have been shown to receive input from the optic tectum; efferents from dorsolateralis posterior were traced to a small triangular wedged-shaped field within the ventromedial portion of the periectostriatal belt.